In recent years, development of technologies for portable electronic devices has been remarkable, and electronic devices such as mobile phones and notebook computers have begun to be recognized as a fundamental technology that supports advanced information society. In addition, research and development of high functionality of such electronic devices have been actively conducted and power consumption of such electronic devices is also increasing in proportion to the high functionality. On the other hand, it is desirable for such electronic devices to be driven for a long time, and a high energy density is necessarily desired for a secondary battery serving as a driving power source. In addition, it is desirable to increase a cycle lifespan in consideration of environmental aspects.
In consideration of a volume and mass occupied by a battery that is built in an electronic device, a higher energy density of the battery is desirable. Currently, since lithium ion secondary batteries have higher voltages and more excellent energy densities than other battery systems, they are built in most devices.
In general, in a lithium ion secondary battery, lithium cobalt oxide is used in a positive electrode, a carbon material is used in a negative electrode, and an operating voltage in a range of 4.2 V to 2.5 V is used. In a single battery, excellent electrochemical stability of a non-aqueous electrolyte material and a separator has a great influence on increasing a terminal voltage to 4.2 V.
A great deal of research for obtaining higher performance and increasing applications of such lithium ion secondary batteries is proceeding. As one example, as a method of increasing a charging voltage, research on increasing an energy density of a positive electrode active material including lithium cobalt oxide and obtaining a high capacity of a lithium ion secondary battery is proceeding.
However, when charging and discharging are repeated at a high capacity, there is a problem of capacity deterioration and a battery lifespan being reduced. In addition, when a battery is used in a high temperature environment, there is a problem of a gas being generated inside the battery and leakage and battery deformation being caused. Therefore, in the related art, a method in which a small amount of LiMn1/3Co1/3Ni1/3O2 is mixed with a positive electrode active material, another material is used to cover a surface, and thus the positive electrode active material is modified has been performed.
In addition, in surface covering of the positive electrode active material, effects of cycle characteristic improvement and thermal stability improvement according to the form of covering have been studied. The following Patent Literature 1 discloses a positive electrode active material that includes a covering layer containing an oxide of Li, Ni, and Mn and in which a concentration of Mn in the covering layer is higher in an outer layer portion than an inner layer portion.
In addition, the following Patent Literature 2 discloses an influence of a concentration of Mn in a material covered with modified LiCoO2 and a covering layer that includes Ni and Mn.
In addition, in the following Patent Literature 3, states of Co and Mg in a covering layer are defined.